L. pneumophila causes Legionaires' disease and Pontiac fever in humans. The potential of L. pneumophila to cause disease has been attributed in part to the presence of the macrophage infectivity potentiator (mip) in this species, as described by Riffard, et al. (1996. Epidemiol. Infect.,17 (3):501-506). The mip protein is a 24 kDa surface protein, which is required for optimal intracellular infection of both protozoa and human macrophages, as discussed by Engleberg, et al. (1989. Infect. Immun., 57 (4): 1263-1270). The sequence of the mip gene appears to be genetically stable and unique to a particular species of Legionella, such as L. pneumophila. Furthermore, the mip gene has been successfully used as a model by Ratcliff, et al. (1998. J Clin. Microbiol.,36 (6): 1560-1567) for a classification scheme for the Legionella genus.
Nucleic acid amplification is a powerful technology, which allows rapid detection of specific target sequences. It is therefore a promising technology for the rapid detection and identification of L. pneumophila. PCR identification of L. pneumophila using sequences within the mip gene has been described by Ramirez, et al. (1996. Clin. Infect. Dis., 24:7-14), Miller, et al. (1993. J Infect. Dis., 168:769-772), Weir, et al. (1998. Am. J Clin. Pathol., 110 :295-300), Bej, et al. (1991. Appl. Environ. Microbiol., 57 (2):597-600), and Koide, et al. (1995. Clin. Infect. Dis., 21:199-201). The oligonucleotide primers of the present invention are applicable to nucleic acid amplification and detection of L. pneumophila.
The following terms are defined herein as follows:
An amplification primer is a primer for amplification of a target sequence by extension of the primer after hybridization to the target sequence. Amplification primers are typically about 10-75 nucleotides in length, preferably about 15-50 nucleotides in length. The total length of an amplification primer for SDA is typically about 25-50 nucleotides. The 3' end of an SDA amplification primer (the target binding sequence) hybridizes at the 3' end of the target sequence. The target binding sequence is about 10-25 nucleotides in length and confers hybridization specificity on the amplification primer. The SDA amplification primer further comprises a recognition site for a restriction endonuclease 5' to the target binding sequence. The recognition site is for a restriction endonuclease which will nick one strand of a DNA duplex when the recognition site is hemimodified, as described by G. Walker, et al. (1992. Proc. Natl. Acad. Sci. USA 89:392-396 and 1992 Nucl. Acids Res. 20:1691-1696). The nucleotides 5' to the restriction endonuclease recognition site (the "tail") function as a polymerase repriming site when the remainder of the amplification primer is nicked and displaced during SDA. The repriming function of the tail nucleotides sustains the SDA reaction and allows synthesis of multiple amplicons from a single target molecule. The tail is typically about 10-25 nucleotides in length. Its length and sequence are generally not critical and can be routinely selected and modified. As the target binding sequence is the portion of a primer which determines its target-specificity, for amplification methods which do not require specialized sequences at the ends of the target the amplification primer generally consists essentially of only the target binding sequence. For example, amplification of a target sequence according to the invention using the Polymerase Chain Reaction (PCR) will employ amplification primers consisting of the target binding sequences of the amplification primers described herein. For amplification methods that require specialized sequences appended to the target other than the nickable restriction endonuclease recognition site and the tail of SDA (e.g., an RNA polymerase promoter for Self-Sustained Sequence Replication (3SR), Nucleic Acid Sequence-Based Amplification (NASBA) or the Transcription-Based Amplification System (TAS)), the required specialized sequence may be linked to the target binding sequence using routine methods for preparation of oligonucleotides without altering the hybridization specificity of the primer.
A bumper primer or external primer is a primer used to displace primer extension products in isothermal amplification reactions. The bumper primer anneals to a target sequence upstream of the amplification primer such that extension of the bumper primer displaces the downstream amplification primer and its extension product.
The terms target or target sequence refer to nucleic acid sequences to be amplified. These include the original nucleic acid sequence to be amplified, the complementary second strand of the original nucleic acid sequence to be amplified and either strand of a copy of the original sequence which is produced by the amplification reaction. These copies serve as amplifiable targets by virtue of the fact that they contain copies of the sequence to which the amplification primers hybridize.
Copies of the target sequence which are generated during the amplification reaction are referred to as amplification products, amplimers or amplicons.
The term extension product refers to the copy of a target sequence produced by hybridization of a primer and extension of the primer by polymerase using the target sequence as a template.
The term species-specific refers to detection, amplification or oligonucleotide hybridization to a species of organism or a group of related species without substantial detection, amplification or oligonucleotide hybridization to other species of the same genus or species of a different genus.
The term assay probe refers to any oligonucleotide used to facilitate detection or identification of a nucleic acid. Detector probes, detector primers, capture probes, signal primers and reporter probes as described below are examples of assay probes.
A signal primer comprises a 3' target binding sequence which hybridizes to a complementary sequence in the target and further comprises a 5' tail sequence which is not complementary to the target (the adapter sequence). The adapter sequence is an indirectly detectable marker selected such that its complementary sequence will hybridize to the 3' end of the reporter probe described below. The signal primer hybridizes to the target sequence at least partially downstream of the hybridization site of an amplification primer. The signal primer is extended by the polymerase in a manner similar to extension of the amplification primers. Extension of the amplification primer displaces the extension product of the signal primer in a target amplification-dependent manner, producing a single-stranded product comprising a 5' adapter sequence, a downstream target binding sequence and a 3' binding sequence specific for hybridization to a flanking SDA amplification primer. Hybridization and extension of this flanking amplification primer and its subsequent nicking and extension creates amplification products containing the complement of the adapter sequence which may be detected as an indication of target amplification.
A reporter probe according to the present invention functions as a detector oligonucleotide and comprises a label which is preferably at least one donor/quencher dye pair, i.e., a fluorescent donor dye and a quencher for the donor fluorophore. The label is linked to a sequence or structure in the reporter probe (the reporter moiety) which does not hybridize directly to the target sequence. The sequence of the reporter probe 3' to the reporter moiety is selected to hybridize to the complement of the signal primer adapter sequence. In general, the 3' end of the reporter probe does not contain sequences with any significant complementarity to the target sequence. If the amplification products containing the complement of the adapter sequence described above are present, they can then hybridize to the 3' end of the reporter probe. Priming and extension from the 3' end of the adapter complement sequence allows the formation of the reporter moiety complement. This formation renders the reporter moiety double-stranded, thereby allowing the label of the reporter probe to be detected and indicating the presence of or the amplification of the target.
The term amplicon refers to the product of the amplification reaction generated through the extension of either or both of a pair of amplification primers. An amplicon may contain exponentially amplified nucleic acids if both primers utilized hybridize to a target sequence. Alternatively, amplicons may be generated by linear amplification if one of the primers utilized does not hybridize to the target sequence. Thus, this term is used generically herein and does not imply the presence of exponentially amplified nucleic acids.